Sunlight associated hyperthermia as a consistent and rapidly developing clinical sign in sheep intoxicated by St John's wort ( Hypericum perforatum )

Objective To assess the usefulness of rectal temperature responses in Australian bred Merino sheep, following the oral administration of Hypericum perforatum (St John's wort), as an early indicator of Hypericum intolerance. Design Thirty‐three Merino ewes were divided into three groups of 11. Each group was dosed with finely ground, dried, flowering growth stage H perforatum plant material at either 5.7, 4.0, or 2.85 g dry plant per kg live weight. This corresponded to 5.3, 3.7 and 2.65 mg hypericin per kg live weight, respectively. Procedure The sheep were dosed with a plant slurry by stomach tube and then exposed to bright sunlight for up to 5 h per day over successive days. Their clinical responses were observed and rectal temperature measured. Results Ingestion of H perforatum followed by exposure to bright sunlight frequently resulted in clinical signs attributable to skin irritation and central nervous effects, including an inappropriate increase in body temperature. A decrease in H perforatum ingestion from 5.7 to 2.85 g dry plant per kg live weight and a corresponding decrease in hypericin ingestion from 5.3 to 2.65 mg per kg live weight, was associated with a decrease in the severity of the clinical signs, including the severity of the hyperthermia. Conclusions The rectal temperature rise in affected sheep is a reliable indicator of the early development of an adverse clinical effect. There appears to be an absolute requirement for exposure to bright sunlight before any effects of H perforatum will develop. A single dose of H perforatum remains potentially effective for up to 4 days. In the small group of Merino sheep tested a tolerance level for H perforatum , eaten at the flowering stage, of<1% (plant wet weight) of body weight and a tolerance level for hypericin of<2.65 mg per kg live weight, were demonstrated.